1. (Field of the Invention)
The present invention relates to a converter adapted to be applied with DC voltage and to provide the boosted output voltage, which is used, for example, with a flash discharge lamp serving as photographic lighting means.
2. (Background Art)
FIG. 6 is a circuit diagram of the conventional photoflash discharge lamp.
This photoflash discharge lamp utilizes a converter including a booster transformer 11 and a switching transistor 12 to boost DC voltage supplied from a battery source 13 and to charge a main capacitor 14 with this boosted voltage.
More specifically, upon closure of a source switch 15, the battery source current flows through emitter-base of the transistor 12 and a resistor 16, thus turning the transistor 12 ON. A capacitor 18 serves to stabilize operation.
The battery current flows through a primary coil 11P of the booster transformer 11 and boosted voltage is generated across a secondary coil 11S.
This boosted voltage is applied to the main capacitor 14 via a diode 17 and thus the main capacitor 14 is charged.
In the course of this charging, the transistor 12 causes the primary coil current to be rapidly increased under a positive feedback effect by a charging circuit comprising the secondary coil 11S, the diode 17, the main capacitor 14 and the emitter-base of the transistor 12
When the primary coil current increases up to a predetermined level depending on the internal resistance of the transistor 12 and the battery source 13, the transistor 12 is turned OFF.
The above-mentioned operation is repeated at first-half stage of charging of the main capacitor 14 and the booster transformer 11 is magnetically saturated before the primary coil current increases up to said predetermined level, since the secondary coil current decreases as the charging of the main capacitor 14 progresses.
Consequently, the transistor 12 is turned OFF due to said magnetic saturation and such operation is repeated until the main capacitor 14 is charged to a predetermined voltage value.
The photoflash discharge lamp circuit includes a trigger circuitry comprising a trigger switch 19, a trigger capacitor 20 and a trigger transformer 21 and, upon closure of the trigger switch 19, a flash discharge tube 22 is applied with excitation voltage.
The flash discharge tube 22 thus excited discharges the main capacitor 14 for light emission.
The booster transformer 11 as the important component of the converter in this photoflash discharge lamp circuit comprises the primary coil 11P and the secondary coil 11S lap wound one on another.
For example, after the secondary coil 11S has been wound by several thousand turns on a flanged bobbin, the primary coil 11P is wound by several ten turns on the outer periphery of the secondary coil 11S. The bobbin on which the coils have been wound is provided with a E-E-shaped or E-I-shaped ferrite core to form a desired small sized transformer.
In view of a fact that the demand for miniaturization of the booster transformer grows day by day, various attempts have already be made to meet such demand, for example, by reducing a bobbin thickness or modifying a configuration of the ferrite core.
However, with the booster transformer of this type, the output voltage usually reaches 300 to 400 volts or higher and the counter electromotive force generated in the secondary coil 11S as the state of the transistor 12 changes from ON to OFF sometimes reaches 1500 volts or higher. With a consequence, excessively reduced bobbin thickness has often caused leak accident between the primary and secondary coils and/or between a low voltage portion and a high voltage portion of the secondary coil.
On the other hand, the configurational modification of the ferrite core is necessarily limited by a predetermined quantity of iron required for the proper function of the core.